Pancreatic cancer is a deadly illness with an extremely low five-year survival rate (only ~5.5%), ranking fourth among the most frequent causes of death from cancer in the United States. New adjuvant approaches to therapy that can complement the limited existing treatment strategies are urgently needed. Gemcitabine is a standard therapy for pancreatic cancer, and there is evidence it can synergize with cellular immunity. Cellular immune responses against tumors are primarily initiated by dendritic cells (DCs), via their presentation of antigens from engulfed tumor cells to T lymphocytes. The localization and numbers of DCs in vivo are determined by the cytokine milieu. The cytokine Flt3L preferentially expands DC1 cells and increases the type 1 T cell response and the numbers of natural killer cells, and it has been shown to have an immunologically mediated anti-tumor effect in several cancer models. The objectives of this project are to optimize the use of Flt3L/gemcitabine against pancreatic tumors in preclinical mouse models, and to establish the mechanistic basis for their effectiveness. Our central hypothesis is that Flt3L, in conjunction wit gemcitabine, will effectively increase the immune response against pancreatic tumors and delay tumor growth. Our rationale for this project is that optimization and understanding of mechanism are necessary steps before a clinical trial of Flt3L/gemcitabine in pancreatic cancer patients is begun. Our Specific Aims are, first, to optimize Flt3L/gemcitabine treatment in pancreatic cancer models. Our hypothesis for this Aim is that the efficacy of Flt3L treatment for pancreatic cancer can be improved by a chemical delivery matrix and that gemcitabine given with Flt3L will have increased anti-tumor effects. Our second Aim is to characterize the mechanism of Flt3L/gemcitabine therapy of pancreatic cancer. Our hypothesis for our second Aim is that Flt3L expansion of DCs and NK cells, plus reduction of immunosuppression and improved antigen cross-presentation due to gemcitabine, increase the immune response against pancreatic tumors. By the completion of this project, it is our expectation that we will have obtained preclinical results on Flt3L therapy for pancreatic cancer that will facilitate the start of a clincal trial.